Method and apparatus for electrochemical recovery of mercury from solutions

ABSTRACT

In embodiments there are disclosed a substantially flat, flow through electrode, electrochemical cells comprising substantially flat flow through cathodes, and methods for electrochemically recovering a metal substantially liquid at room temperature.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to U.K. 1012711.6 filed on Jul. 29,2010 and the disclosure is incorporated herein by reference.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH OR DEVELOPMENT

NOT APPLICABLE

REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAMLISTING APPENDIX SUBMITTED ON A COMPACT DISK

NOT APPLICABLE

BACKGROUND OF THE INVENTION

1. Description of Prior Art

In prior art systems, when an ore is roasted, the necessary processingsolutions may release gases, which may then be treated to recovermercury and particulates, the resulting scrubber bleed solution may thenbe treated to separate the liquids from the solids, which are then takento a leaching process. Then the liquids may be mixed with Zinc, in a mixtank, and the solids and liquids are separated and through a chemicalreaction mercury is extracted as a mercurous chloride precipitate andthe supernatant solution is disposed of.

PCT/GB00/01388 to Gilroy, filed Apr. 12, 2000, describes a cylindricalelectrochemical cell comprising a cylindrical flow through cathode forthe electrochemical deposition of mercury and gallium. The cathodesurrounded by a cylindrical anode, the anode and cathode compartments ofthe cell are separated by a proton conducting membrane.

U.S. Pat. No. 5,292,412 to Pitton, Issued on Mar. 8, 1994, describesmetal alloy porous electrodes, and electrochemical cells whereinsolution flow is directed across the face of the electrodes.

The abstract of SU1668483 to Barmashenko, discloses the use of a carbonfelt cathode to electrochemically precipitate mercury.

The abstract of SU1760780 to Vsesoyuznyj discloses a flow through carbonfiber cathode to deposit mercury.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a method and apparatus forelectrochemically removing mercury from solutions.

In an embodiment there is disclosed a substantially flat, flow throughelectrode.

In an alternative embodiment the electrode may have a surface area of atleast about 500 m2 per 1 m2 of geometric surface.

In alternative embodiments there are disclosed a cathode according to anembodiment and a carbon felt electrode according to an embodiment.

In an alternative embodiment there is disclosed an electrochemical cellcomprising an electrode according to an embodiment.

In an alternative embodiment there is disclosed a cathode according toan embodiment for electrochemically depositing from solution a metalsubstantially liquid at room temperature.

In an alternative embodiment the metal is mercury.

In an alternative embodiment there is disclosed an electrochemical cellcomprising a cathode according to an embodiment and an anode, whereinthe cathode and the anode may each comprise substantially flatgeometrical surfaces and the substantially flat cathode and anodegeometrical surfaces are mutually opposed and are substantiallyuniformly distanced.

In an alternative embodiment the distance between the electrodegeometrical surfaces is less than about 2 cm and the cathode and theanode are in direct fluid contact.

In an alternative embodiment distance between the anode and cathode isless than about 1 cm.

In an alternative embodiment the electrochemical cell further comprisesa solution inlet positioned to direct at least a portion of the solutionto flow through the cathode.

In an alternative embodiment the electrochemical cell is configured sothat substantially all of the solution flows through the cathode.

In an alternative embodiment the electrochemical cell may furthercomprise a collector for collecting the metal under gravity induced flowwhen the metal is electrochemically deposited at the cathode.

In an alternative embodiment there is disclosed an apparatus fortreating a scrubber bleed solution, the apparatus comprising theelectrode according to an embodiment.

In an alternative embodiment there is disclosed an apparatus fortreating a scrubber bleed solution, the apparatus comprising theelectrochemical cell according to an embodiment.

In an alternative embodiment there is disclosed an apparatus comprisinga plurality of electrochemical cells according to embodiments.

In an alternative embodiment there is disclosed a method for recoveringfrom solution a metal substantially liquid at room temperature, themethod comprising collecting metal electrochemically deposited at acathode according to an embodiment.

In an alternative embodiment there is disclosed a method for recoveringmercury from solution, the method comprising collecting mercuryelectrochemically deposited at a cathode according to an embodiment.

In an alternative embodiment there is disclosed a method for recoveringfrom a solution a metal substantially liquid at room temperature, themethod comprising the step of electrolytically depositing the metal at aflow through cathode positioned in the solution, wherein the solutiondirectly contacts both the cathode and a corresponding anode.

In an alternative embodiment the cathode and the anode each have asubstantially flat geometric surface and the anode geometric surface andthe cathode geometric surface are mutually opposed and substantiallyuniformly separated by a distance of less than about 2 cm.

In an alternative embodiment the cathode may have a surface area of atleast about 500 m2 per 1 m2 of geometric surface.

In an alternative embodiment the cathode may be a carbon fibre cathodeor may be a carbon cathode or may comprise carbon, pyrolyzed parylene C(PCC), .carbon foam, carbon nanofoam, carbon coatings, carbon films,carbon pastes, carbon beads, carbon microbeads, carbon microtubes,carbon nanotubes, graphite, graphene, pyrolytic graphite, highlyoriented pyrolytic graphite, randomly oriented graphite, carbon black,carbon fiber, evaporate a-C, a-C:H, pyrolyzed photoresist film, borondoped diamond, or N-doped amorphous tetrahedral carbon.

In an alternative embodiment the current density between the anode andthe cathode may be less than about 10V per m2 of geometrical cathodesurface.

In an alternative embodiment the solution may be a scrubber bleedsolution.

In an alternative embodiment the metal may be mercury.

In an alternative embodiment there is disclosed a continuous flow methodaccording to an embodiment.

In an alternative embodiment there is disclosed an electrochemical celladapted to receive an electrode according to an embodiment andcomprising a solution inlet adapted to direct an electrolyte to flowthrough the electrode.

In an alternative embodiment there is disclosed an electrochemical celladapted to receive the electrode according to an embodiment andcomprising a solution inlet adapted to direct an electrolyte to flowthrough the electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an electrode according to a firstembodiment.

FIG. 2 is a schematic representation of a general process comprising thebleed and destruction systems according to an embodiment.

FIG. 3 is a schematic representation of the recovery system according toan embodiment.

FIG. 4 is a side view of the interior of an electro-chemical cell inaccordance with a first embodiment.

FIG. 5 is an end view of the interior of an electrochemical cellaccording to FIG. 4, taken at right angles to FIG. 4.

FIG. 6 is a top plan view of the interior of the cell according to FIGS.4 and 5.

FIGS. 7A and 7B are enlarged views of portions of FIG. 4.

FIGS. 8A and 8B are enlarged views of portions of FIG. 5.

FIG. 9 is a cut away sectional view of a second embodiment

FIG. 10 is a sectional view of the embodiment according to FIG. 10.

DETAILED DESCRIPTION OF THE INVENTION Description of Embodiments

Definitions:

In this disclosure the term “pore” or “pores” means any interstice,space, passage, channel, opening, perforation, cavity or similarstructure by means of which a substance may pass through a structure.

In this disclosure the term “porosity” or “porousness” refers to theratio or relative relationship of the number or volume of pores in asubstance, structure or mass relative to the total geometrical volume orgeometrical area as defined by the gross external dimensions of thesubstance or structure or mass of the substance or structure. Porosityis generally referred to as a ratio of the volume of the pores relativeto the gross geometrical volume of the structure. In particularembodiments electrodes or substances may be porous and may be highlyporous. A porous electrode, which may be highly porous and may be acathode, may be or may be greater than, about 20%, great than about 25%,greater than about 30%, greater than about 35%, greater than about 40%,greater than about 45%, greater than about 50%, greater than about 55%,greater than about 60%, greater than about 65%, greater than about 70%,greater than about 75%, greater than about 80%, greater than about 85%,greater than about 90%, greater than about 95% porous or more, or may bein a range delimited by values of about 40%, about 45%, about 50%, about55%, about 60%, about 65%, about 70%, about 75%, about 80%, about 85%,about 90%, about 91%, about 92%, about 93%, about 94%, about 95%, about96%. about 97%, about 98%, about 99% about 99% or greater porosity.

In this disclosure the term “highly porous” indicates a porosity ofgreater than about 50%, greater than about 55%, greater than about 60%,greater than abut 65%, greater than about 70%, greater than about 75%,greater than about 80%, greater than about 85%, greater than about 90%,greater than about 95% or more, or and may indicate a range of porositydelimited by values of about 70%, about 75%, about 80%, about 85%, about90%, about 95%, about 95%, about 97%, about 98%, about 99%, or greaterporosity.

In this disclosure the term “felt” means a structure formed from mattedor compressed fibers and “carbon felt” means a felt or structure formedfrom matted or compressed carbon fiber. The manufacture, handling andpurchase of suitably formed felts, including carbon felts, will bereadily understood and achieved by those skilled in the art.

In this disclosure, electrodes may be constructed in any conventionalshapes or materials all of which will be readily identified, understoodand adopted by those skilled in the art. In particular embodimentselectrodes may be anodes or cathodes or both, and may be constructed topresent an enlarged surface area. relative to the geometrical volume orsurface of the electrode material itself. By way of example but withoutlimitation, a cathode may be porous and may be constructed to present alarge geometrical surface area by shaping the cathode in the form of oneor more flattened or curved plates, sheets or membranes, or as aplurality of wires, threads, fibers or tubes, or the electrode maycomprise any other structure or conformation suited to present anincreased surface area for contact with an electrolyte which may be ascrubber solution. In particular embodiments a cathode may comprise anelectrically conductive felt or reticulated material, non-limitingexamples of which include a conductive felt, mesh or net of any kind andexamples include carbon felt or reticulated carbon. In embodiments thecarbon felt may be formed of carbon fibres, which fibres may be formed,for example, by the carbonization and/or graphitization of syntheticpolymer fibres, for example, polyacrylonitrile or ester fibres. The feltmay be formed from a pad of such carbon fibres and the pad may becompressible. In embodiments, the fibres may suitably have a diameter ofthe order of about 6 to 8 microns, especially about 6 microns. But inalternative embodiments a range of alternative materials will be readilyidentified and adopted by those skilled in the art and where used fibersmay have diameters of, or of greater than or less than about 0.001,0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.2,0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,12, or more microns, micrometers, or millimeters. It will be understoodthat an electrode having the desired properties may be made in a formother than a felt or fibrous structure or may comprise a coated fibrousor non-fibrous structure and may comprise a solid or semi solidstructure with pores provided therethrough and may comprise or maycomprise an electrolytic surface that comprises one or more of carbon,pyrolyzed parylene C (PCC), .carbon foam, carbon nanofoam, carboncoatings, carbon films, carbon pastes, carbon beads, carbon microbeads,carbon microtubes, carbon nanotubes, graphite, graphene, pyrolyticgraphite, highly oriented pyrolytic graphite, randomly orientedgraphite, carbon black, carbon fiber, evaporate a-C, a-C:H, pyrolyzedphotoresist film, boron doped diamond, N-doped amorphous tetrahedralcarbon and other materials which will be readily apparent to thoseskilled in the art who will readily select between them and adapt theircompositions and structures for particular purposes. It will beunderstood that electrodes may comprise or have associated therewith ormay be shaped to cooperate with, suitable supporting frames, clips,mountings or other structures which may maintain the structure,integrity or position of the electrodes. Such structures and frames mayinclude but are in no way limited to internal and external frames ofmetals, plastics, carbon, and any other material of sufficient strengthand rigidity to maintain the desired geometrical shape of the electrodein operation. This may also comprise the provision of a net or meshinterwoven with the electrode material or covering and containing theelectrode material.

In this disclosure the statement that an electrode or a geometricalsurface of an electrode is flat or substantially flat or generally flatmeans that at least one geometrical surface of the electrode isgenerally planar, or has only limited curvature or divergence from aplane. It will be recognised however that, while such a surface may begenerally flat, some degree of irregularity or lack of smoothness may bepermissible in embodiments, and those skilled in the art will readilydetermine the degree of smoothness or the tolerance that is necessary ordesirable for acceptable or desirable performance of electrodes,electrode pairs and electrochemical cells according to embodiments.

In this disclosure reference to a geometric surface or geometric surfacearea of an electrode or structure means the external surface or grossexternal shape of the electrode or structure and is to be distinguishedfrom more general references to surface and surface area of anelectrode, which, unless the context otherwise requires, indicate thepotentially reactive surface of the electrode at which electricalcontact between the electrode and an electrolyte may occur. Thus wherean electrode is porous, the total surface area of the electrode includessurface area presented within the pores. By way of example and notlimitation, if an electrode has the general conformation of a squareplate, then the geometrical surface area of the electrode will bedefined by the areas of such square faces and any plate edges. It willtherefore be apparent that in the case of a porous electrode, the totalsurface area of the electrode will be substantially greater than thegeometrical surface of the electrode. It will be further understood thatreference to the geometrical volume of an electrode refers to the volumedefined by the external dimensions of an electrode.

In this disclosure an indication that an electrode has a high surfacearea to volume ratio means that the area of electrode surface that ispotentially available to electrically contact an electrolyte is highrelative to the external geometrical volume of the electrode or area, asdefined by the external dimensions of the electrode. It will be furtherunderstood that in an electrode with a high surface area to volumeratio, the ratio of total surface for contact with the electrolyte togeometrical surface of the electrode will be greater than 1:1. Inembodiments such a high surface area to volume ratio may be, or may begreater than, about 1,000:1, 2,000:1, 3,000:1, 4,000:1, 5,000:1,6,000:1, 7,0001, 8,000:1, 9,000:1 or 10,000:1, 15m000:1, 20,000:1,25,000:1. Similarly the ratio may be between about 50:1 and about1000:1, about 100:1 and about 1000:1, about 200:1 and about 1000:1,about 300:1 and about 1000:1, about 400:1 and about 1000:1, about 100:1and about 900:1, about 100:1 and 800:1, about 100:1 and about 700:1,about 100:1 and about 600:1. Likewise, in embodiments, a high surfacearea to volume ratio may have any value within these ranges.

In particular embodiments the geometrical shape and size of an electrodemay be of any desired dimensions, but in particular embodiments thedimensions will be chosen to reduce the current density between opposedanodes and cathodes. Thus in particular embodiments the geometricalsurface area of a cathode or the opposed geometrical surfaces of acathode/anode pair may be chosen to be as large as practicable to reducethe unit current flow between electrodes over a given area. Thus inparticular embodiments any one of the edges of the substantially flatsurface of a cathode or anode may be greater than about 1, 1.5, 2, 2.5,3, 3.5, 4, 4.5, 5, 5.5, 6, or more meters or greater than about 1, 2, 3,4, 5, 6, 7, 8, 9, 10 or more feet. In embodiments the geometricalsurface area of a cathode or anode may be greater than about 5, 6, 7, 8,9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26,27, 28, 29, 30, 35, 40, 45, 50, 100 or more square meters.

In particular embodiments a cathode may have about 20 m2 of geometricsurface area, although greater or lesser surface areas and particulargeometrical dimensions can be selected and will be readily selected fromby those skilled in the art. Where a cathode has a geometrical surfaceof about 20 m2 then it may provide a total cathode surface of about250,000 m2 but different ratios of total surface to geometric surfacewill be readily created and selected amongst by those skilled in theart. It will be appreciated that the current flow per unit area maydepend primarily on the opposed geometrical surfaces of opposed orpaired electrodes rather than by the entire external surfaces of theelectrodes.

In this disclosure an “electrochemical cell” also referred to as a“cell” means any device designed to pass electrical current between ananode and a cathode through an electrolyte liquid. In embodiments theelectrolyte may be a solution, may be a scrubber bleed solution, or maybe derived from a scrubber bleed solution. In one embodiment, a typicalelectrochemical cell for treating a scrubber bleed solution may havedimensions of about 5 ft.×4 ft.×6 ft although it will be understood thatany convenient dimensions may be chosen to suit operational requirementsand to accommodate desired electrode dimensions, and such possibledimensions will be readily selected from by those skilled in the art tosuit particular operational requirements.

In embodiments a cell may be operated with a potential difference acrossthe cell of about 10V, and the gap between cathode and anode may be lessthan about 2 cm, and in some cases about 1 cm or less than about lcm.Alternative voltages will be readily selected by those skilled in theart to suit particular cell properties and dimensions, and by way ofexample, in alternative embodiments a potential difference of up to orgreater than about 1V, 2V, 3V, 4V, 5V, 6V, 7V, 8V, 9V, 10V, 11V, 12V,13V, 14V, 15V, 16V, 17V, 18V, 19V, 20V or greater may be applied to thecell.

In particular embodiments, the rate of flow of an electrolyte, which maybe a scrubber bleed solution, through an electrochemical cell may berelatively low and may be less than about 50,000 cm2/sec/m2, and may beless than about 45,000, 40,000, 35,000, 30,000, 25,000, 20,000, 15,000,14,000, 13,000, 12,000, 11,000, 10,000, 9,000, 8,000, 7,000, 6,000,5,000, 4,000, 3,000, 2,000, 1,000 or fewer cm2/sec/m2. It will beunderstood that in alternative embodiments the flow rate of electrolytethrough an electrochemical cell of an embodiment may be above or belowabout 1,000, 2,000, 3,000, 4,000, 5,000, 6,000, 7,000, 8,000, 9,000,10,000, 11,000, 12,000, 13,000, 14,000, 15,000, 16,000, 17,000, 18,000,19,000, 20,000, 21,000, 22,000, 23,000, 24,000, 25,000, 26,000, 27,000,28,000, 29,000, 30,000, 31,000, 32,000, 33,000, 34,000, 35,000, 36,000,37,000, 38,000, 39,000, 40,000, 45,000, 50,000 cm3/sec/m2 and thoseskilled in the art will readily adjust the flow rate to suit particularpurposes and to achieve desirable performance parameters with particularcell geometries and electrolyte compositions. In particular embodimentsthe pressure difference between the inlet and outlet of the cell may bevery low and may be close to zero.

In this disclosure the term “hydrogen potential” of an electrode, andparticularly a cathode means the reduction potential of the electrode.In embodiments cathodes may have high hydrogen potentials and may havehydrogen potentials of up to or greater than or about 2000 mV (twothousand millivolts). Those skilled in the art will understand that thehydrogen potential of an electrode may be modified by the electrolyte inwhich it is immersed.

In this disclosure, reference to a metal that is “liquid at roomtemperature” or is “substantially” or “normally” liquid at roomtemperature, means that the metal is or, is normally, or is in part,liquid at room temperature or temperatures approximating roomtemperature, or at a temperature of greater than about 1 oC, 2 oC, 3 oC,4 oC, 5 oC, 6 oC, 7 oC, 8 oC, 9 oC, 1O oC, 11 oC, 12 oC, 13 oC, 14 oC,15 oC, 16 oC, 17 oC, 18 oC, 19 oC, 20 oC, or 21 oC, and less than about95 oC, 90 oC, 85 oC, 80 oC, 75 oC, 70 oC, 65 oC, 60 oC, 55 oC, 50 oC, 45oC, 40 oC, or 45 C. In particular embodiments the metal may be Mercury.It will be further understood that the stipulation that a metal isnormally or substantially liquid at room temperature, does not therebyrequire that the operation of an electrochemical cell according to anembodiment is carried out at a temperature that approximates roomtemperature. Thus for a reaction in aqueous solution, a cell maypotentially operate at any temperature between the freezing and boilingpoints of the aqueous solution, or of the metal substantially liquid atroom temperature, subject to any operational limits that may be imposedby a user for safety reasons or for any other reasons. All suchadjustments and determinations will be readily made by those skilled inthe art. Similarly, in any embodiment wherein an electrochemical cell isoperated using an electrolyte that is not an aqueous solution, the cellmay be operated at any temperature that is consistent with the safeoperation of the cell.

In this disclosure the term “substantially liquid” or “normally liquid”or “liquid” where used to describe a metal, indicates the ability of themetal to flow, and encompasses the full range of possible viscositiesthat may be compatible with the operation of a cell or electrodeaccording to embodiments disclosed herein.

In this disclosure paired anodes and cathodes of embodiments may presentmutually opposed geometrical surfaces that are separated by a distance.In particular embodiments the opposed anode and cathode surfaces may beseparated by a distance of less than about 2.0 cm, 1.9 cm, 1.8 cm, 1.7cm, 1.6 cm, 1.5 cm, 1.4 cm, 1.3 cm, 1.2 cm, 1.1 cm, 1.0 cm, 0.95 cm,0.90 cm, 0.85 cm, 0.80 cm, 0.75 cm, 0.70 cm, 0.65 cm, 0.60 cm, 0.55 cm,0.50 cm. In embodiments such separation distance may be substantiallyconstant over the opposed area of the electrode surfaces. By“substantially constant” is meant that the separation distance may varyfrom point to point to an extent that does not prevent the effective ordesired operation of an electrochemical cell according to embodiments.It will be appreciated that if the separation of the opposed platesbecomes less uniform, this may affect the performance of the cell inways that will be readily understood and managed by those skilled in theart who will readily determine acceptable parameters for an electrodepair for particular applications and will understand when an electrodeis in need of repair or replacement. In particular embodiments where anelectrode has a geometrical surface of about 20 square meters, then theseparation distance between opposed anode and cathode faces may bebetween about 0.5 cm and about 2 cm.

In this disclosure the statement that an electrolyte or a solution or aliquid flows or may, or may in part, flow “through” an electrode, whichmay be a cathode, indicates that the electrode is, or is in part, porousso that the electrolyte, solution or liquid is able to pass through suchpores from one side of the electrode to another side of the electrode,without having to flow around the geometrical surface of the electrode.

In this disclosure “scrubber bleed solution” means the solutiongenerated from processing of off-gases from a range of processes,including roasting of ores and tailings. A scrubber bleed solution maytypically have an initial mercury concentration of up to 500 ppm.However, in particular embodiments it will be understood that a scrubberbleed solution may contain any concentration of Mercury or of any otherliquid normally or substantially liquid at room temperature and maycontain more or less than about 50 ppm, 100 ppm, 150 ppm, 200 ppm, 250ppm, 300 ppm, 350 ppm, 400 ppm, 450 ppm, 500 ppm, 550 ppm, 600 ppm, 650ppm, 700 ppm, 750 ppm, 800 ppm, 850 ppm, 900 ppm, 950 ppm, 1000 ppm orgreater of either alone or in combination with any other metals or otherchemical components. Further it will be understood that an electrolyteor electrolyte solution may contain similar or greater or lesserconcentrations of Mercury.

In this disclosure reference to a “continuous flow”, “recycling” or“recirculation” of electrolyte means that the electrolyte iscontinuously circulated through one or more electrochemical cells, andmay be supplemented, replenished, added to, diluted or otherwisemodified during such recirculation process. This is to be distinguishedfrom a batch process wherein the electrochemical cells or theirassociated containing structures are drained, or the electrolyticprocess temporarily or permanently halted after the processing of eachbatch of electrolyte. The use of a continuous flow process may mean thatthe extraction of mercury or other metal substantially liquid at roomtemperature may be continued for extended periods, for example inembodiments the processing may be continued for 24 hours a day, 7 days aweek, or for such period as may be necessary or desirable thus allowingthe continuous processing of solution in response to its source. Acontinuous flow process may be desirable for the processing of arelatively dilute electrolyte. It will be understood that even whenelectrolyte is processed in a continuous flow manner, it may benecessary or desirable to shut the process down from time to time topermit maintenance and adjustments to the apparatus or process.

In this disclosure, in alternative embodiments, a low current densitymeans a current density of less than about 1, 2, 3, 4, 5, 6, 7, 8, 9,10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, 30, 35, 40, 45, or 50Vper square meter.

In this disclosure, a “collector” “collection device” or the like,unless the context requires otherwise, refers to a device or designfeature for collecting deposited metal normally liquid at roomtemperature may comprise any suitable structure or structures, includingtrays, pipes, channels, grooves, valves or other devices orconstructions. Those skilled in the art will readily recognise andconstruct a wide range of suitable collectors and adapt and implementthem into embodiments of the subject matter disclosed.

A statement in this disclosure that an electrode pair are in directelectrical contact through an electrolyte, or are both in direct contactwith the electrolyte, indicates that the two electrodes are notseparated by a membrane or other divider, such as a proton conductivemembrane and that the same electrolyte solution is in direct physicalcontact with both electrodes.

In this disclosure a scrubber solution may be derived from any sourceincluding but not limited to the roasting of mineral ores includingtailings. In particular but non limiting embodiments the ore may be goldore.

In this disclosure an ore or a mineral ore, refers to any ore, rock ortailings, that may be processed to extract components therefrom.

Embodiments are hereafter generally described with reference to FIGS. 1through 10.

First Embodiment

In a general form of the first embodiment, generally designated 400 andillustrated in FIG. 1 there is disclosed a substantially flat, flowthrough electrode assembly which may be a cathode.

FIG. 1 shows a simplified diagram of an electrode for use in the firstembodiment, which electrode may be a cathode. The electrode 400 maycomprise a porous electrode body 412. The porous electrode 412 may beconfined and supported by supporting grids 416 and 418. Grid 416comprising vertical components 411 and horizontal components 423 andgrid 418 comprising vertical components 415 and horizontal components414. Metal feeder sheet 413 may be secured between porous cathodematerial 412 and grid 418 or may be positioned external to the grid. Itwill be understood that a variety of alternative arrangements of thecomponents and a variety of alternative materials may be selected forparticular requirements and will be readily understood and implementedby those skilled in the art. When mounted in a suitable electrochemicalcell it will be understood that additional supporting structures,gaskets, and wiring to apply an electrical current to the electrode, mayall be incorporated in the electrode. For simplicity and clarity thesedetails are omitted from FIG. 1 but will be readily understood by thoseskilled in the art and such structures and the arrangement of anelectrode within a cell are further explained elsewhere in thisdisclosure with reference to FIGS. 4 through 8.

It will be seen that the feeder sheet 13 comprises a plurality ofopenings 433 distributed upon the sheet to allow electrolyte to accessand flow through the electrode 412. As will be seen particularly fromFIG. 1, the geometric surface of the electrode body 412 is defined byspatial dimensions X, Y, Z. This is to be distinguished from the actualtotal surface area of the electrode which may include pores, andnotwithstanding the geometric surface of the electrode, the electrodemay have a total surface area of at least about 500 m2 for each 1 m2 ofgeometric surface and may be a cathode. The porous electrode 412 may bemade of or may comprise carbon felt or may comprise other materials. Inan alternative embodiment the electrode may be a cathode and may be usedfor depositing from solution a metal substantially liquid at roomtemperature, and the metal may be mercury.

A cathode 412 according to the first embodiment may be porous orotherwise constructed to present a large surface area and may be in theform of an electrically conductive felt or reticulated material,especially a carbon felt or reticulated carbon. A carbon felt orreticulated carbon may provide a high porosity particularly in excess of90%; the high porosity provides a high cathode surface area and inembodiments this may provide an increased reaction rate and may besuitable for use with dilute solutions. Where used, carbon felt may beformed of carbon fibres, which may be formed form the carbonizationand/or graphitization of synthetic polymer fibres, for example,polyacrylonitrile or ester fibres. In particular embodiments the fibresmay suitably have a diameter of the order of 6 to 8 microns, especiallyabout 6 microns. Since a pad of fibres may be mechanically fragile, itmay be mechanically supported in a frame or assembly which serves tohold the felt in a planar state. The felt may be supported undercompression so as to have a planarity whereby a constant gap orseparating distance is maintained between the cathode and an opposedanode. The cathode should, of course, be resistant to materials andconduction which may be present in the cell where it is used, whichmaterials and conditions may be extreme and may include exposure to thepresence of chlorine and chloride ions, heat, cold, acidity, alkalinity,oxidation and reduction conditions. Carbon felt and reticulated carbonare generally suited to these requirements. In embodiments the materialof the cathode may have a high hydrogen over potential which may allowmercury or other metal normally or substantially liquid at roomtemperature to be deposited in preference to liberation of hydrogen atthe cathode 412.

FIGS. 4 through 8 illustrate an electrochemical cell generallydesignated 100 comprising an electrode according to the general designof the first embodiment. As will be seen from FIGS. 4 through 8, in usea cathode 12 according to embodiments may be paired with an anode 10 ina suitable electrochemical cell 100. An anode 10 is generallyessentially impermeable to the electrolyte solution, which may be or maycomprise scrubber bleed solution or a treated extract or derivativethereof. In particular, an anode 10 may comprise a dimensionally stableelectrode and may typically have a core of titanium sheet coated with ametal oxide, for example, one or more oxides of tantalum, iridium andplatinum. A range of alternative suitable materials will be readilyidentified and implemented by those skilled in the art. Withoutlimitation, any forms of inert or relatively inert and conductive metaland/or metal oxide may be suitable for use as or in anodes according toembodiments. The anode 10 suitably may have a low oxygen overpotentialsuch that hydroxide ions are discharged liberating oxygen in preferenceto chloride ions releasing chlorine.

The cathode 12 and the anode 10 each may comprise substantially flatgeometrical surfaces and the substantially flat cathode and anodegeometrical surfaces 210, 212 are mutually opposed and are substantiallyuniformly distanced 214. In particular embodiments the distance betweenelectrode geometrical surfaces 210, 212 is less than about 2 cm and thecathode 12 and the anode 10 are in direct fluid contact, and inembodiments the distance 214 is about lcm or less than about 1 cm.

Electrochemical cells 100 according to embodiments may further comprisea solution inlet 102 positioned to direct at least a portion of theelectrolyte solution to flow through the cathode 12. The cell, which maycomprise one or more collecting trays 100, may be configured so thatsubstantially all of the solution flows through the cathode 12.Electrochemical cells may also comprise one or more collectors 220 forcollecting a metal substantially liquid at room temperature undergravity induced flow when the metal is electrochemically deposited atthe cathode 12. The inflow passage 17 may be angled or shaped so thatany deposited metal is guided to collect at a collection point 222, orany trays 220. The collector structure itself may have any suitabledesign and may be a simple drain to allow the deposited metal to flowout of the cell 100 to be harvested in a suitable container.

A cathode and anode may form an electrolysis electrode assembly 106 in acell 100. In particular embodiments of a cell 100 electrode assemblies106, each assembly having a cathode 12 and an anode 10 whose opposedfaces are separated by a distance 214 therebetween. A flow path for flowof solution being treated extends from an inlet 102 to an outlet 104 ofthe cell. The flow path provides a contact time between the flowingsolution and the electrode assemblies 106 sufficient for deposition ofthe mercury metal at the porous cathode 12. The flow path may, inparticular, comprise an inflow passage 17 and an outflow passageextending across the cell in opposed generally parallel arrangement,with the electrode assemblies 106. These may be provided or in multiplesand in embodiments a plurality of electrode assemblies 106 is arrangedextending in spaced apart relationship between the inflow 102 andoutflow 104 passages and generally perpendicular thereto. A plurality ofdiscrete branch passages between the electrode assemblies maycommunicate with the inflow passage 17 into the plurality of branchpassages and from there through an adjacent porous cathode 12 into thegap 214 between such cathode 12 and its anode 10, the gap forms a gappassage communicating with the outflow passage and the solution flowsalong the gap passage against the anode 10 and into the outflow passage2 and from there exits from the cell 100.

Suitably a gas passage is maintained as small as possible, for example,1 cm or less. In this way a plurality of discrete treatment flow pathsis formed within the cell 100 thereby maximizing the electrochemicallyactive surface area of the cell per cell volume.

In a further variant of the embodiment, there is disclosed apparatus fortreating a scrubber bleed solution, the apparatus comprising theelectrode according to embodiments. In embodiments the apparatus maycomprise a plurality of electrochemical cells. The electrochemical cellsmay be connected in series or in parallel and the scrubber bleedsolution may be recycled through them with periodic or ongoing additionof fresh scrubber bleed solution.

In further detail, the embodiment illustrated in FIG. 1 and FIGS. 4through 8 and is described as follows. Electrochemical cell 100 has aninlet 102 and an outlet 104. Inlet 102 communicates with and inflowpassage 17 and outlet 104 is in communication with an outflow passage 2.

A plurality of electrode assemblies 106 is housed in cell 100 includinga pair of end electrode assemblies 108, 110 and a plurality ofintermediate electrode assemblies 112. Each of end assemblies 108 and110 includes a cathode assembly 114 supporting a porous cathode 12spaced form a dimensionally stable anode 10.

Each of intermediate electrode assemblies 112 includes a pair of cathodeassemblies 114 each supporting a porous cathode 12 spaced from a singledimensionally stable anode 10, therebetween. Flow passages 16 aredefined between adjacent intermediate electrode assemblies 112 andbetween intermediate electrode assemblies 112 and end electrodeassemblies 108 and 110, respectively. The flow passages 16 communicatewith in-flow passage 17 but are closed adjacent out-flow passage 2 bycaps 4. The electrode assemblies 106 are pressed together as an assemblybetween a pair of end supports 160 comprising inner end plates 1,suitably of PVC, and outer end plates 18, suitably of steel. Cell 100includes a plurality of cathode supports 19 each comprising cathodefeeder 3 suitably in box section of mild steel sheet. With particularreference to FIGS. 7A and 7B which show an enlargement of details A andB of FIG. 4, each cathode assembly 114 comprises a frame 6 and a porouscathode 12 suitably a carbon fibre felt supported between a grid 116 anda grid 118. A metal feeder sheet 13, conveniently an expanded stainlesssteel sheet having a plurality of orifices, is supported between cathode12 and grid 118 but alternatively the grid 116 may be positioned betweenthe feeder 13 and electrode body 12 so long as suitable electricalcontact is maintained between electrode body 12 and feeder sheet 13.Grid 116 is conveniently of PVC and comprises a plurality of spacedapart vertical members 11 and a plurality of spaced apart horizontalmembers 23.

Grid 118 is conveniently of mild steel and comprises a plurality ofspaced apart vertical members 5 and a plurality of spaced aparthorizontal members 14. The grids 116 and 118 hold the porous cathode 12with a required degree of planarity, in spaced relationship with anode10.

With particular reference to FIGS. 4 and 5, and FIGS. 7A and 7B outergaskets 7 are disposed between feeder sheets 13 and frame 6 adjacentout-flow line 2 and outer gaskets 15 are similarly disposed adjacentin-flow passage 17. Gaskets 8 and 9 are disposed between opposed sidesof anode 10 and grids 116 in the vicinity of out-flow passage 2 andin-flow passage 17. The gaskets 7, 8, 9 and 15 are suitably of neoprenebut a range of other suitable materials will be readily identified,selected from, and used by those skilled in the art.

With particular reference to FIGS. 7A, 7B and 8A and 8B, the cathodesupports 19 provide electrical contacts and are suitably of mild steel.A plurality of insulation and anode supports 20 suitably of PVC, houseanode feeds 21, suitably of copper or other electrically conductivemetal, connected to each anode 10.

With particular reference to FIG. 8, a mesh 22 suitably of polypropyleneis disposed between porous cathode 12 and grid 116. With particularreference to FIG. 7, a plurality of flow paths 120 in parallel areidentified by flow line arrows.

Thus in one embodiment as shown generally in FIGS. 4 and 5, the cell 100consists of ten (10) cathodes 12 and eleven (11) anodes 10. The cathodes12 and anodes 10 are pressed together between the end plates 1 and theresulting assembly is compressed between end plates 18 which thusprovide the rigidity which cell 100 needs to ensure a uniform separationof each anode 10 and is associated cathode 12.

Each cathode 12 has opposed porous surfaces, the geometric integrity andplanarity of which is maintained by there being contained between thethree-dimensional grids 116 and 118.

The cathode assembly 114 may comprise a cathode feeder 3 suitably of boxsection mild steel, which acts as a frame and a means of distributingelectrical current to grid 118. The expanded metal feeder sheet 13 iswelded onto the vertical member 5 of grid 118. Feeder sheet 13 acts as acurrent distributor to the porous cathode 12 and as a physical constrainto ensure that the porous cathode 12 retains a uniform thickness. Theporous cathode 12 is pressed against the feeder sheet 13 by thethree-dimensional grid 116, suitably of PVC; the polypropylene mesh 22between grid 116 and cathode 12 ensures the planarity of the surface ofcathode 12 facing anode 10.

Grid 116 is attached to frame 6, suitably of PVC, which is suitablybolted through feeder sheet 13 to a corresponding frame 6 of theadjacent cathode assembly 114. Gasket 7 is interposed between frame 6and feeder sheet 13. The electrode assemblies 106 are supported by thecathode feeders 3 which act as electrical contacts and by supports 44which also conduct current to the electrode.

The anode 10 suitably consists of a titanium sheet coated with one ormore metal oxides to produce a dimensionally stable anode. In particularembodiments, suitable anode types include those produced by El-techCorporation of Cleveland, Ohio. The current to the anodes 10 isdistributed by anode feeds 21, suitably four copper strips riveted oneach face at either side of an anode 10. The anode 10 is supported bythe insulation and anode supports 20 on either side which alsoelectrically insulate the anode 10 from the cathodic bussing.

In operation the scrubber solution after it has been passed through amercury recovery system is now termed scrubber bleed solution. Thescrubber bleed solution enters the electrochemical cell 100 through theinlet 102 and passes into the in-flow passage 17.

The solution passes through the cell 100 in parallel flow paths 120 thedistribution of the solution through the cell 100 is governed by thepressure drop associated with each possible flow path 120. The onlysignificant pressure drop in the system is that across the face of theporous cathode material of cathode 12. This ensures that each electrodereceives a similar flow of solution. Any imbalance is self correcting asan increase in flow through any electrode will result in greaterdeposition and a consequent rise in the pressure across the electrode.

From the in-flow passage 17 the solution passes up into the flow passage16 where its exit is blocked at the top by cap 4. The solution exitsthrough the faces of the cathode 12 through the grid 118, the expandedfeeder sheet 13, the porous cathode 12, the polypropylene mesh 22 andinto the grid 116.

The solution passes up the grid 116 between the anode and the cathode 12and into the out-flow passage 2 from where it exits via outlet 104 inthe endplates 1 and 18.

Second Embodiment

A second embodiment of an electrochemical cell is shown in FIGS. 9 and10 and is generally designated 300. In the embodiment the cell 300comprises body 301 holding a single anode 310 and a single cathode 312separated by a distance 314. It will be seen that the interior 304 ofthe cell forces the electrolye which may be or derive from a scrubberbleed solution and may enter the cell through an inflow 302 leading intoinflow chamber 317, to flow through the cathode 312 and around the anode310 passing to outflow chamber 320 and then exiting the cell 300 throughoutflow 304. It will be appreciated that the cell 300 incorporatessuitable brackets and mountings to hold the cathode 312 and anode 310 inplace and comprises an electrical supply to apply a current through theelectrodes and electrolyte. Mountings are generally designated 350 and352, but for simplicity the detail of such mountings and of any powersupply is omitted from FIGS. 1, 9 and 10. A range of suitable methodsand materials for the mounting and application of a potential differenceto the electrodes will be readily apparent and will be readilyimplemented by those skilled in the art.

The bottom 306 of cathode 312 is engaged by a drainage channel orcollector 340 with a collection point 342 for any deposited metal thatis substantially liquid at room temperature. In operation, aselectrolyte flows through the cathode 312, which may be a carbon feltcathode or of any other construction, such as the constructionillustrated in FIG. 1 and described above, the metal, which may bemercury, is deposited at the cathode, and flows under gravity downthrough the openings in the cathode to accumulate in collection channel340, whence it is collected for further use at the collection point atend 342 of the channel 340. The collection channel may be of anysuitable size but in an embodiment may be about 1.25 inches in diameteror may be narrower or wider.

It will be appreciated that in this embodiment, it may be of particularimportance to restrict the flow rate of solution through the cell inorder to prevent undue pressure on the cathode structure. While one formof the second embodiment is illustrated with reference to FIGS. 9 and 10it will be appreciated that applied current, electrolyte flow rate,inlet and outlet positions, and other parameters may be adjustedaccording to required performance parameters in ways readily apparent tothose skilled in the art. Examples of possible modifications areprovided in the description of the first embodiment.

Third Embodiment

In an third embodiment there is disclosed a method for recovering from asolution a metal substantially liquid at room temperature, the methodcomprising the step of electrolytically depositing the metal at a flowthrough cathode. In one embodiment of the third embodiment here isdisclosed apparatus and a method for treating a scrubber bleed solutionwhich may be generated from off gases, or from the processing of mineralores. A general embodiment of a method for treating off gases from oreprocessing, is shown in FIG. 2 and FIG. 3. It will be seen that in somecases, for clarity, features shown in one of the FIGS. 2 and 3 may beomitted in the other.

With reference to FIG. 2, when an ore is roasted, the necessaryprocessing solutions 500 may release gases, which may then be treated510 to recover mercury and particulates, the resulting scrubber bleedsolutions 660 may then be treated at 670 to separate the liquids fromthe solids, which are then taken to a leaching process 671. Then theliquid is mixed with necessary conditioning reagents 545, in a mix tank550, then the conditioned solution is passed through an electrochemicalcell 680 so that mercury liquid 681 can be recovered, and outflowsolution 580 may be returned to the process cycle 700 which may beoperated on a continuous flow basis.

With reference to FIG. 3, which shows the general process according toFIG. 2 in the context of associated quench towers, scrubbers and thelike, gas from ore roasting, 599 is introduced to a quench tower 600along with introduced treatment solutions 602, bleed solution 605 isdrawn off for processing and output 602 from the quench tower 600. Theoutput from quench tower 600 is conveyed to a particulate scrubber 610,mixed with treatment solutions 612, and bleed liquids drawn off at 615while output 621 from the scrubber 610 flows to the sulphur dioxidescrubber 620 to be mixed with treatment solutions 622. Bleed solution625 is drawn off and the output 621 from the scrubber 620 flows to firstmercury scrubber 630. Treatment solutions 632 is introduced and theoutput from the first mercury scrubber 630 flows to second mercuryscrubber 640 to be mixed with treatment solution 642. The output 641from the second mercury scrubber 640 flows to tails scrubber 650 fortreatment with solutions 652, gas is vented at 651 and bleed solution655 is removed for processing.

The bleed solution from the first and second mercury scrubbers 630, 640,is combined in a common feed 660 leading into a separation tank 670where solids 671 are removed for processing.

The scrubber bleed solution from this separation process flows to anelectrochemical cell 680, where mercury or other metals substantiallyliquid at room temperature are electrochemically deposited and areharvested. In the case of mercury metal this is typically in solutionsthe form of mercuric ions. The electrochemical cell 680 may beconstructed or operated according to the first or second or otherembodiments. In flowing through the porous cathode, mercuric ions aredischarged electrochemically within the porous structure and the treatedsolution flowing from cell is thus poor in mercury metal ions. As theporous cathode becomes loaded with deposited mercury metal, the mercurywill coalesce and become a free flowing liquid gathering at the base ofthe electrode. A suitable collector for collecting the liquid mercury isprovided for at the base of the cathode or cathodes or more generally inthe base of the cell 680. The scrubber bleed solution to the cell can betemporarily interrupted without terminating the operation of the process

A surge tank 690 is provided for the used electrolyte solution, make upsolution or additives 691 are added as needed, and the resultingsolution 700 is returned to scrubbers 600, 610, 620, 630, 640, 650 asnecessary or desirable. Thus in operation the scrubbing process may beoperated on an essentially or substantially continuous flow basis. In acontinuous operation scrubber bleed solution may be continuously cycledfrom the roaster or combustion process scrubber to the electrochemicalcell and treated solution recycled to the process.

In one embodiment, a typical cell 680 for treating scrubber bleedsolution may have dimensions of about 5 ft.×4 ft.×6 ft. The cathode mayhave about 20 m2 geometric surface area, while the porosity of thecathode may provide a total cathode surface of about 250,000 m2.

The cell may be operated with a potential difference across the cell ofabout 10V, the gap between cathode and anode may be less than about 2cm, and in some cases about 1 cm or less than about lcm. Such a cell maybe operable treat a scrubber bleed solution flow of 20,000 cm3/sec/m2.The scrubber bleed solution may typically have an initial mercuryconcentration of up to 500 ppm.

It will be appreciated that the specifics of dimensions, flow rates,surface areas, volume and other parameters of a cell can be readilymodified and adapted by those skilled in the art all consistent with theembodiments disclosed herein. For example cells may be made larger,smaller, or shaped for particular applications, the numbers andarrangements of electrodes may be modified and a wide range of otheradaptations will be recognised and implemented by those skilled in theart.

The rate of flow of scrubber bleed solution may be maintained low and inparticular embodiments may typically be between about 5,000 cm3/sec/m2to 20,000 cm3/sec/m2, depending on the concentration of mercury metal.At high concentrations of mercury metal the flow rate may preferably bemaintained at the at the lower end of the 5,000 cm3/sec/m2 to 20,000cm3/sec/m2 range. In one embodiment the pressure drop between the inflowpassage and the outflow passage is effectively zero, so that thepressure drop through the porous cathode governs the flow rate, thisflow rate being governed by the porosity.

Alternative Embodiments

In an embodiment there is disclosed a method and apparatus for theremoval of a metal which may be mercury from a solution which may be ascrubber solution, whereby the resulting solution can be recycled. Inthe case of a scrubber solution it may be recycled to the mill process.In some embodiments the scrubber solutions may be created by theroasting of ore. In an alternative embodiment the mercury iselectrochemically deposited and subsequently collected as metallicmercury. In a further alternative embodiment the scrubber solution mayflow through a porous cathode and across the surface of an anode of anelectrochemical cell. In a further alternative embodiment embodiment apotential difference is maintained between the cathode and the anode toeffect electrochemical deposition of the mercury metal in the porouscathode. In a further alternative embodiment the porous cathode may havea high electrochemically active surface area per unit volume and inembodiments this be achieved by employment of a cathode material of highporosity in excess of 90%. In embodiments the cathode can be loaded witha high level of mercury metal which is collected by allowing the mercuryto exit the electrode at its base by gravity induced flow. In a furtheralternative embodiment the cathode materials may have a substantiallyevenly distributed or substantially homogenous, or substantially uniformporosity. In embodiments the porosity should permit loading of thecathode with a mercury level of more than 0.5 g/cm3.

The embodiments and examples presented herein are illustrative of thegeneral nature of the subject matter disclosed and are not limiting. Itwill be understood by those skilled in the art how these embodiments canbe readily modified and/or adapted for various applications and invarious ways without departing from the spirit and scope of the subjectmatter disclosed. The subject matter hereof is to be understood toinclude without limitation all alternative embodiments and equivalents.Phrases, words and terms employed herein are illustrative and are notlimiting. Where permissible by law, all references cited herein areincorporated by reference in their entirety. It will be appreciated thatany aspects of the different embodiments disclosed herein may becombined in a range of possible alternative embodiments, and alternativecombinations of features, all of which varied combinations of featuresare to be understood to form a part of the subject matter claimed.Particular embodiments may alternatively comprise or consist of orexclude any one or more of the elements disclosed.

What is claimed is:
 1. A substantially flat, flow through electrode. 2.The electrode according to claim 1 wherein said electrode has a surfacearea of at least about 500 m² per 1m² of geometric surface.
 3. A carbonfelt electrode according to claim
 1. 4. An electrochemical cellcomprising the electrode according to claim
 1. 5. A cathode according toclaim 1 for electrochemically depositing from solution a metalsubstantially liquid at room temperature.
 6. An electrochemical cellcomprising a cathode according to claim 5 and an anode, wherein saidcathode and said anode each comprise substantially flat geometricalsurfaces and said substantially flat cathode and anode geometricalsurfaces are mutually opposed and are substantially uniformly distanced.7. The electrochemical cell according to claim 6 wherein said distancebetween said electrode geometrical surfaces is less than about 2 cm andsaid cathode and said anode are in direct fluid contact.
 8. Theelectrochemical cell according to claim 5 further comprising a solutioninlet positioned to direct at least a portion of said solution to flowthrough said cathode.
 9. The electrochemical cell according to claim 5further comprising a collector for collecting said metal under gravityinduced flow when said metal is electrochemically deposited at saidcathode.
 10. An apparatus for treating a scrubber bleed solution, saidapparatus comprising the electrode according to claim
 1. 11. A methodfor recovering mercury from solution, the method comprising collectingmercury electrochemically deposited at a cathode according to claim 1.12. A method for recovering from a solution a metal substantially liquidat room temperature, the method comprising the step of electrolyticallydepositing said metal at a substantially flat flow through cathodepositioned in said solution and wherein said solution directly contactsboth said cathode and a corresponding anode.
 13. The method according toclaim 12 wherein said cathode and said anode each have a substantiallyflat geometric surface and wherein said anode geometric surface and saidcathode geometric surface are mutually opposed and substantiallyuniformly separated by a distance of less than about 2 cm.
 14. Themethod according to claim 12 wherein said cathode has a surface area ofat least about 500 m² per 1 m² of geometric surface.
 15. The methodaccording to claim 12 wherein the cathode is a carbon fibre cathode. 16.The method according to claim 12 wherein the current density betweensaid anode and said cathode is less than about 10V per m² of geometricalcathode surface.
 17. The method according to claim 12 wherein saidsolution is a scrubber bleed solution.
 18. The method according to claim12 wherein said metal is mercury.
 19. A continuous flow method accordingto claim
 12. 20. An electrochemical cell adapted to receive theelectrode according to any of claims 1 through 3 and comprising asolution inlet adapted to direct an electrolyte to flow through saidelectrode.